1. Field of the Invention
The present invention relates to a transmission method in a case where a functionally-distributed base station is used in a radio access network of a mobile communication system, and particularly relates to a handover technique for keeping communication when a terminal is moved.
2. Description of the Related Art
Use of mobile phones is now expanded to not only voice communication, but also a field requiring a broad band such as access to WWW or video phones. To respond to this, so-called the third-generation method represented by the W-CDMA (Wideband Code Division Multiple Access) method or the MC-CDMA (Multicarrier CDMA) method is being introduced.
As one of methods for responding to increased demand, reduction of cells is given, in which a cover area of a base station is narrowed and thus increased number of base stations are disposed. In the reduction of cells, a cell that is an area allowing communication between the base station and a terminal of a mobile phone is narrowed, and the capacity of the base station is decreased to reduce cost, so that increased number of base stations are disposed. This reduction of cells has the following two problems.
As a first problem, since the cell is narrowed, the number of cells, through which a terminal passes during moving a certain distance, is increased, and consequently frequency of handover processing, or frequency of changing a base station as a communication object by a terminal in communication, is increased. This increases signaling load on base stations in the overall radio access network or a wireless network control apparatus that controls the base stations. Particularly in the W-CDMA system, site diversity (diversity handover) is performed, wherein a terminal communicates with a plurality of base stations at the same time in a period while the terminal is situated in a place where cells of the plurality of base stations are overlapped, so that communication quality is improved in a radio zone. Therefore, hardware resources for transmission of all the base stations to be communicated with the terminal are consumed compared with a case that the handover processing in which the base station is simply changed is performed. Since the wireless network control apparatus has a feature of accommodating a plurality of base stations, the number of installed wireless network control apparatus is extremely smaller than the number of base stations. Therefore a line (called Iub in 3GPP) between the wireless network control apparatus and the base station is typically long compared with length of a physical line between a core network of the mobile phone and the wireless network control apparatus. At that time, when bands of network between the wireless network control apparatus and the base stations are consumed by site diversity, a higher-speed line is necessary between the wireless network control apparatus and the base station, and consequently cost is increased by a level corresponding to the larger length of the line.
As a second problem, when the number of small-scale base stations is increased under a multimedia traffic environment where various amounts of bands are used for each of users, fragmentation of used hardware resources for each of base stations occurs, and consequently substantial usability of the base stations is deteriorated. For example, when hardware resources corresponding to 9 channels are used in a base station that can accommodate voice call corresponding to 16 channels, even if packet call requiring hardware resources corresponding to 8 channels is generated, the base station can not accommodate the packet call because of insufficient hardware resources. This is the fragmentation of hardware resources. While reduction in usability due to such fragmentation is small in the case of large-scale base stations only, in the case of the small-scale base stations, such a situation that a service, which consumes many hardware resources, can not be accommodated due to such fragmentation of hardware resources may increase.
JP-A-2001-45534 describes a base station in which load on the line between the wireless network control apparatus and the base stations is reduced. The base station described in this patent publication has a configuration where a transmission section that performs signal processing associated with a transmission channel such as baseband processing in the base station is separated from an antenna section that transmits and receives a radio signal, and converts the signal into a wired signal to be used within the base station. In the configuration, since the antenna section is realized at low cost compared with a small-scale base station, an area to be covered by a cell can be expanded inexpensively. By collecting hardware resources of the transmission section in large quantities, an advantage of efficiently using the hardware resources is given.
FIG. 15 shows a block diagram of a radio access network (RAN) in the case that the functionally-distributed base stations are introduced in the W-CDMA method. The radio access network of the W-CDMA method is described as TS (Technical Specification) 25. 401 “UTRAN Overall Architecture” in 3GPP (3rd Generation Partnership Project). According to the specification, a wireless network control apparatus that controls base stations in the radio access network and the base stations exist in the radio access network of the W-CDMA method.
FIG. 15 shows a system block diagram in the case that the functionally-distributed base stations are used for the W-CDMA system. While an example where two sets of main base stations and sub base stations are disposed is shown for simplification here, the number of the main base stations or the sub base stations is not limited to two sets. The sub base station converts a down spread signal from the main base station into a radio signal. On the other hand, regarding an up radio signal from a terminal, the sub base station outputs the signal as the spread signal without conversion to a main base station connected thereto. While the main base stations and the sub base stations are in one-to-one correspondence respectively in FIG. 15, actually a plurality of sub base stations are disposed for one main base station. This increases capacity of hardware resources of the main base station, consequently loss due to fragmentation of the hardware resources can be reduced.
When the terminal performs the site diversity with a plurality of cells of sub base stations, in the case that the all the sub base stations belong to the same main base station, macro diversity combining (MDC) may be performed, wherein the main base station combines the up radio signals from the terminal. In this case, bands in which the line between the wireless network control apparatus and the main base station is consumed are the same as in the case that MDC is not performed. When the radio access network is configured only by the small-scale base stations without using such functionally-distributed base stations, MDC is performed in the wireless network control apparatus. Therefore, since the radio signals from the terminal to the wireless network control apparatus is transmitted through all the small-scale base stations as objects of the site diversity, the signals are increased by the number corresponding to the number of the base stations as the objects of the site diversity compared with the functionally-distributed base stations.
In FIG. 15, a wireless network control apparatus 1501 has a function of controlling devices in the radio access network. A first main base station 1502a and a second main base station 1502b are controlled by the wireless network control apparatus 1501, and perform signal processing such as base-band processing of a signal from the terminal and transmission of the signal to the wireless network control apparatus 1501. The first main base station 1502a and the second main base station 1502b have cells having different areas respectively. In the following description, the first main base station 1502a and the second main base station 1502b are named generically as a main base station 1502. The main base station 1502 communicates with the wireless network control apparatus 1501 in the same format as in a typical base station (Node B in the 3GPP standard). Sub base stations 1503a, 1503b are connected to the main base stations 1502a, 1502b, and convert analog radio signals into wired signals and then transmit the signals to the main base stations 1502a, 1502b, respectively. In the following description, the sub base station 1503a and the sub base station 1503b are named generically as a sub base station 1503.
A terminal 1504a and a terminal 1504b are shown as terminals that perform radio communication. In the following description, it is assumed that the terminal 1504a is moved and arrives at a position of the terminal 1504b. A cell (a) is a cell of the first sub base station 1503a, and a cell (b) is a cell of the second sub base station 1503b. 
FIG. 16, FIG. 17 and FIG. 18 show internal configurations of the wireless network control apparatus 1501, main base station 1502, and sub base station 1503 respectively. In FIG. 16, a base station communication section 1601 communicates with the main base station 1502. The base-station communication section 1601 is defined as the Iub interface in UTRAN in 3GPP. A UTRAN control section 1602 performs management and control of nodes or terminals in the radio access network. A general control section 1603 performs control of the wireless network control apparatus 1501 including communication with a core network or operational management.
In FIG. 17, a wireless-network-control-apparatus communication section 1701 communicates with the wireless network control apparatus 1501. The wireless-network-control-apparatus communication section 1701 is defined as the Iub interface in UTRAN in 3GPP. A baseband processing section 1702 performs baseband modulation of a transmission channel. A radio link control section 1703 performs control of a radio link that is a line from the wireless network control apparatus 1501 to the main base station 1502. A sub-base-station communication section 1704 communicates with the sub base station 1503.
In FIG. 18, a main-base-station communication section 1801 communicates with the main base station 1502. A transmission/reception control section 1802 performs start/stop of transmission or change of transmission destination based on instructions of the main base station 1502. A radio communication section 1803 communicates with the terminals 1504a, 1504b using radio signals.
FIG. 19 and FIG. 20 show a data configuration used in the UTRAN control section 1602 of the wireless network control apparatus 1501. As seen in FIG. 19, a cell that is communicates with the terminal 1504a is shown as Active Set. In this example, the wireless network control apparatus 1501 stores a cell ID (a) in PLMN (an abbreviation of Public Land Mobile Network) indicating a cell (a) as Active Set.
FIG. 20 shows a base-station correspondence table 2001. The base-station correspondence table 2001 stores cell IDs in PLMN, names of main base stations, and cell-IDs in RAN of all the cells as control objects in the main base stations for each of the cells. In FIG. 20, since it is assumed that the terminal does not perform the site diversity, the number of elements of the cell-IDs in RAN is one each. In an example in the related art, all the cell-IDs in PLMN uniquely correspond to the main base stations.